Stainless ferrite-austenitic steel

ABSTRACT

A stainless ferrite-austenitic steel of the following composition: Percent C0.02-0.08 Si maximum of 0.5 Mn maximum of 1.0 N maximum of 0.06 Cr24-27 Ni5-7 Mo1.3-1.8 AND COLUMBIUM IN SUCH A CONTENT THAT THE RATIO Cbrest/C is at least eight, Cbrest referring to the total columbium content minus the content of columbium required for binding of the nitrogen present in steel to columbium nitride, and C referring to the total carbon content, the remainder being essentially iron, has a high resistance to intercrystalline corrosion.

United States Patent 1191 Hellner et al.

[ June 19, 1973 [54] STAINLESS FERRITE-AUSTENITIC STEEL Primary Examinerl-lyland Bizot [75] lnventors: Lars lvar Hellner; Nils Erik Allan Atmmey -Hane & Baxley Hede; Hans Elof Johansson, all of Karlskoga, Sweden ABSTRACT. [73] Assigneezr Aktiebolaget Bofors', Bof Sweden A stainless ferrite-austenitic steel of the following composition: [22] Filed: May 12, 1969 [21] Appl. No.: 823,744 percent 0.02-0.08 Si Maxirnum of. I 0.5 [30] Foreign Application Priority Data g ggrfiz g gg 2 May 16, 1968 Sweden 6611/68 o .7 24 2;.

.......................................................... 1. .3-l.8 [52] US. Cl. 7 5/l28 G, 75/128 W MO I [51] Int. Cl. C22c 39/20 .7 1 [58] Field of Search 75/128 W, 128 G and Columbillm in Such a n n hat he ratio rest/C v I t g is at least eight, rest referring to the total columbium [56] Referen es Cited content minus the content of columbium required for 1 UNITED STATES PATENTS binding of the nitrogen present in steel to columbium 3 337 331 8 1967 L b 757128 w itride, and C referring to the total carbon content, the 34998O2 311970 i g a 75/128 w remainder being essentially iron, has a high resistance 2"590074 4/1971 Blo om 75/128 G to intercrystamne corrosion- Claim, 2 Drawing Figures |ooo-- I min.

PATENIEDM v 3.740.213

sum 2 OF 2 l N VEN TORS LARS 044R HEL LIVER N/LS ERIK ALLAN HA'DE HANS ELQ JOHANSSON H MBMW A r rams rs STAINLESS FERRITE-AUSTENITIC STEEL The present invention relates to a method of producing a stainless ferrite-austenitic steel with a high resistance to intercrystalline corrosion.

In cases when a combination of good corrosion resistance, high strength, a high degree of toughness and good malleability have been desired, ferrite-austenitic steels have hitherto been used containing approx.

0.1 percent carbon 27 percent chromium 5 percent nickel and 1.5 percent molybdenum which corresponds to a steel according to SIS 2324. However, it has proved that steel of said type in many cases, e.g. in whale-oil separators, where a relatively aggressive medium is combined with different operating temperatures, is subject to very substantial intercrystalline corrosion, and is therefore unsuitable. It has been possible to show this intercrystalline type of corrosion on a laboratory scale by boiling with a 3' percent sodium chloride solution saturated with silver chloride. Through such laboratory investigations it has been established that the'above-mentioned steel according to SIS 2324 is extremely sensitive to intercrystalline corrosion at heat treatments at approx. 650 C, even at relatively short exposure times of the magnitude of approx. 1 minute. This has the result that, even if water quenching is used, after heat treatment at approx. 1000C., it cannot be avoided, in practice, that such a sensitivity to intercrystalline corrosion takes place with said type of steel, particularly if it is a question of largesized parts, such as globes for separators.

Through the present invention ithas quite surprisingly proved to be possible to eliminate this sensitivity to intercrystalline corrosion in stainless ferriteaustenitic steels, and still, at the same time, to retain the good malleability, the high degree of general corrosion resistance, and the good physical properties.

The method of producing stainless ferrite-austenitic steel with a high resistance to intercrystalline corro-- Percent C max. 0.12

Si max. 1.0 Mn max. 2.0 N max. 0.1 Cr 20-30 Cb 4-8 -Mo 1-2.s

and the remainder iron, together with the usual impurities in the type of steel and accessorial components. Said ferrite austenitic steel can advantageously contain:

Percent C 0.02-0.08 Si max. 0.5 Mn max. 1.0 N max. 0.06 Cr 24-27 Ni 5-7 Mo l.3l.8

The invention also relates to steels produced according to any of the above-mentioned methods.

It has, indeed, previously been proposed to use a columbium additive in order to avoid intercrystalline corrosion, but it has then only been a question of conditions in connction with welding, and it has then been considered that an addition of columbium corresponding to eight times the carbon content would be sufficient to eliminate the sensitivity to intercrystalline corrosion. However, both information given in literature and own trials have shown that such an addition of columbium cannot eliminate the sensitivity to intercrystalline corrosion, and it must therefore be regarded as being extremely surprising that such considerably improved resistance to intercrystalline corrosion has been obtained through the present invention.

The invention will now be described in more detail with reference to studies of a number of different steels, the chemical composition of which is given in a table 1. Steel No. 1 according to this table corresponds to the above-mentioned type of steel according to SIS 2324, and the other steels have been graduated according to increasing value of the ratio Cb lC. In addition to the chemical composition given in table 1, in the thirdcolurnn from the right, the calculated carbon content which is not bound as columbium carbide is indicated, consideration then having been taken to the fact that the columbium is primarily bound to the nitrogen present in the steel, forming columbium nitride. The

values for the ratio between the total columbium content and carbon, (CB J/C and the ratio Cb /C have also been indicated in table 1.

Steels 1 and 9 in said table have been investigated as regards corrosion at different isothermal treatments, using a boiling 3 percent sodium chloride solution saturated with silver chloride. The results of these investigations will be noted from FIG. 1, in which curve A indicates the temperatures (T) in "C and the exposure time (t)' in minute at which a weight loss of 5 percent has occurred in steel 1, while curve B indicates a weight loss of 10 percent in steel 1. Curve Ccorres ponds to a weight loss of 5 percent in steel 9 and curve D, finally, a weight loss of 10 percent for steel 9. In the zones located to the left of curves A and C, the corrosion has mainly taken place in the form of pitting. Within the zones between curves A and B and C and D,respectively, the corrosion shown for the steels has consisted of both pitting and intercrystalline corrosion. To the a right of curvesB and D the corrosion hasmostly been of the intercrystalline nature. From FIG. 1 it will be noted that in steel according to $118 2324 the intercrystalline corrosion reaches its maximum value at approx. 650C, and that, as previously mentioned, even at such short exposure times as approx. 1 minute, a substantial intercrystalline corrosion occurs. From FIG. 1 it will further be noted that a steel according to the present invention (No. 9) has considerably lower sensitivity to intercrystalline corrosion and that, contrary to the case of steel according to SIS 2324, no practical'difficulties are encountered with steel according to the present invention in achieving such rapid quenching after heating to approx. 1000C that, even with large dimensions,

one does not risk passing the zone, that gives sensitivity to intercrystalline corrosion.

The steels indicated in table 1 have been investigated with regard to corrosion at different heat treatments, and in table 2 the type of corrosion is indicated for ratio Cb,,,,,/C 7.8, i.e. where some of the steels that have been investigated, and at the same time the ratios Ch /C and Ch /C. From table 2 it will be clearly noted that heavy intercrystalline corrosion and pitting takes place even at such high Cb z/C ratios as 12 (steel 4) in which however, the ratio Cb, ,,/C only amounts to 1. For steel 6 with the ratio Ch /C Of 22 and Cb JC of 7, it is still possible to note certain, although relatively moderate, intercrystalline corrosion together with pitting. As regards steels 7, 8 and 9, all of which have values above eight of the ratio Cb /C the corrosion mainly takes place in the form of pitting. Although steel 7 has a lower value of the ratio Cb /C than steel 6, the intercrystalline corrosion, however, is considerably less in steel 7 than in steel 6. In other words, table 2 clearly shows that it is the ratio Cb /C that is decisive for the nature of the corrosion while, on the other hand, the ratio Cb /C in this connection is of minor importance. Similar investigations of corrosion conditions when boiling in a sodium chloride solution saturated with silver chloride for 24 hours are shown in FIG. 2. As will be noted from FIG. 1, the intercrystalline corrosion depends to the highest degree upon the heat treatment, and in order to isolate the intercrystalline corrosion, in FIG. 2, the difference between the weight loss (V in a condition with quenching only and the weight loss in said quenched condition in combination with isothermal treatment has been plotted on the vertical Y axis. This isothermal treatment has taken place at 600C for 2 hours as well as at 700C for 2 hours ([1). The proportions between columbium and carbon have been shown on the horisontal X axis, the point where the 1 mole CB corresponds to 1 mol carbon then having been chosen as origo. At this point there is thus neither free carbon nor free columbium, but these two elements have been completely bound to each other in the form of columbium carbide (CbC). As previously mentioned, when calculating the content of Ch consideration has been taken to the fact that the nitrogen present in the steel takes care of the columbium before the carbon in the form of columbium nitrate (CbN). In reality, a certain quantity of columbium can also be fixed in the form of columbium oxide or other slag containing co lumbium, but since, as is usually the case in steel of this type, aluminium is present in a content of a few hundredths percent, such a binding of columbium to oxygen is of no practical significance. For the value of the ratio Ch /C that is less than 7.8, the columbium will of course not be sufficient to bind all the carbon, and there will be certain content of free carbon in the steel, and this content has been plotted along the right-hand part of the horizontal X axis as percent free carbon (C,). For the value of the ratio Cb /C exceeding 7.8, there will be a surplus of columbium and the steel will have a certain content of free columbium, and this content has been plotted on the left-hand part of the horizontal X. axis as percent free columbium (Cb From FIG. 2 it will be perfectly clear that for the steels in which the ratio Cb /C are considerably below eight (steels No. l to 5), the weight loss caused by the intercrystalline corrosion is considerably greater than for the steels (7 l l) in which the ratio cb,,,,/c exceeds the value of 8.

However, the addition of columbium should not be too great, as the undesirable tendency towards a phase formation already found in steel according to 81$ 2324 increases with the addition of columbium. This tendency towards incerased 0' phase formation at an in creased addition of columbium will be noted from table 3, in which the 0' phase formation for most of the steels listed in table 1 has been investigated at two different heat-treatment conditions, heating to 975C for 1 hour and cooling in water, followed by sensitizing at 700 for 2 hours as well as the same quenching annealing at 975for 1 hour followed by sensitizing for 2 hours, but this time at 800C. As will be noted from table 3, no a phase could be found at the lower sensitizing temperature, while, on the other hand, at the higher sensitizing temperature, the content of 0' phase that could be ascertained increased as the value of the ratio Cb /C increased. However, this a phase formation can be counteracted by keeping the ratio Cb, ,,/C between 8 and 20 and at the same time keeping the silicon content low, appropriately at approx. 0.5 percent.

Table 3 also gives the austenite content of the steel after quench annealing from heating for 1 hour at 975C. It then proved that steel No. 2 had a considerably higher austenite content than the other steels, and this high austenite content also proved to be less favourable, as the malleability was substantially impaired, at the same time as the yield point was lowered. The austenite content should be between 5 and 30 percent, particularly between 15 and 25 percent. A low austenite content primarily has an unfavourable influence on the impact strength, and in order to obtain a suitable austenite content in the steel, the contents of nickel, manganese and chromium must be appropriately balanced. Moreover, chromium is necessary in order to give the steel the good corrosion properties desired, but its content should not exceed approx. 30 percent, as the tendency towards 0' phase formation will then increase. Also the presence of molybdenum in the steel leads to increased corrosion resistance, and the molybdenum content should appropriately be approx. 1.5 percent. The carbon content can be kept low for two reasons; the low carbide content consequently obtained has a favourable influence on the malleability and impact strength and, moreover, a lower carbon content will reduce the content of columbium additive required in order that the ratio Cb /C shall amount to at least eight. In order to obtain carbon contents below 0.04 percent, however, sepcial metallurgical proce dures are required, which make the steel considerably more expensive, and in the present case, approx. 0.05 percent carbon therefore seems to be appropriate. Also the nitrogen content should be kept low, in order to limit the columbium additive required.

Table 4, finally, shows some physical properties of the steels investigated. These physical properties have been determined according to the ISO Recommendations No. R82 (1959) and R 148 (1960). As will be noted from table 4, the physical properties have mainly been retained, in spite of the columbium content, and through the present invention it has therefore become possible to achieve a stainless ferrite austenitic steel that has physical properties which are practically equal to those of the previously used ferrite-austenitic steel according to SIS 2324, but which has considerably less sensitivity to intercrystalline corrosion.

Structure after various Heat-Treatment Conditions TABLE 1.CHEMICAL COMPOSITIONS OF STEEL A LLOYS INVESTIGATED carbon content not Content in percent ofbougdJ (2 3s Cbm Cbruat o si Mn Cr N1 M Cb N percent 0 0 Steel Number:

TABLE 2 TABLE 4 V v I Physical Properties of Steel Alloys investigated Type of Corrosion after Heat-Treatment 975C, l h Yield Ummate Comm Impact 600C, 2 h, Water. Steel point tensile tion tion strength 20 strength (0' KCV No. kp/mm kp/mm kpm Steel Ch /C C rul/C Corrosion l 57 72 27 4 9 No. 5 2 47 70 34 71 20 l 0 0 Heavy intercrystalline corrosion 3 48 67 31 69 I5 pitting 5 52 68 3t 63 16 6 0 I 0 s2 66 25 05 4 l2 1 25 7 s9 59 24 6 22 7 Moderate intercrystalllne 3 67 22 59 corrosion pitting 9 53 g 23 g 7 l7 Mainly p g. 10 54 07 25 51 8 39 l 9 36 27 il 55 55 23 x 54 l. A stainless ferrite-'austenitic steel with a high resis- 30 tance to intercrystalline corrosion having the following TABLE 3 composition, in percent by weight,

Percent C 002-008 Si maximum of .5 Mn maximum of L0 N maximum of 0.06 Cr 24-27 Ni 5-7 Mo l.3-l.8

and columbium in such content that the ratio Cb lC is at least eight, Ch referring to the total columbium content minus the content of columbium required for binding of the nitrogen'present to columbium nitride, and C referringto the total carbon content, the remainder being essentially iron. 

